A high-speed Universal Serial Bus (USB) communications protocol, such as, for example, USB version 3.0 (USB3 or SuperSpeed USB), serializer/deserializer (SerDes) operating at 5 Gigabits per second (Gbps) requires extremely low power dissipation because USB is generally intended for portable devices where power consumption is a critical concern. During normal operation, USB 3.0 devices are typically in sleep (standby) mode most of the time in order to conserve power. Periodically, however, the USB 3.0 device is required to wake up for communication with a host device to which the USB device is connected.
While a connected USB device is in low power (e.g., sleep or standby) mode, it generally conveys its presence to the host device by sending a “ping” message (also referred to as “pinging”). The ping message is used to test the presence of an active client at the other end of the connection. A ping message is often sent at regular intervals if no other activity is detected coming from a given connection. If the connection fails to respond to a ping message within a prescribed period of time, that connection is closed.
In order to send a ping message, however, a link layer in the connected device is required to wake up the entire clock multiplication unit (CMU) and transmitter to send a two cycle “ping clock.” This standard approach of sending out-of-band signals is costly, at least from a power perspective, and is therefore undesirable, particularly for mobile devices which seek to minimize power consumption.